Increasingly stringent statutory provisions relating to the permissible pollutant emissions of internal combustion engines which are disposed in motorized vehicles make it necessary to adopt various measures by means of which the pollutant emissions can be reduced. One starting point in this endeavor is to reduce the pollutant emissions generated during the combustion process of the air/fuel mixture. The formation of soot in particular is greatly dependent on the preparation of the air/fuel mixture in the respective cylinder of the internal combustion engine. In order to achieve a very good fuel mixture preparation, fuel is increasingly metered under very high pressure. In the case of diesel internal combustion engines, the fuel pressures reach as high as 2000 bar. Injection valves having a piezo actuator as the final control element are becoming increasingly widely accepted for applications of this kind. Piezo actuators are characterized by very short response times. Injection valves of this type are thus possibly suited for metering fuel several times over within one working cycle of a cylinder of the internal combustion engine fuel.
A particular good fuel mixture preparation can be achieved if one or more pre-injections, which are also referred to as pilot injections, are performed prior to a main injection, with possibly a very small fuel mass being required to be metered for the individual pre-injection. Precise control of the injection valve is very important in particular for cases of this type.
A method for detecting injection events of an injection valve having a piezoelectric actuator is known from WO 01/63121. The injection valve comprises an injector body having a control chamber to which is assigned a control valve which controls the fuel pressure in the control chamber. The piezoelectric actuator acts on the control valve. A voltage is applied to the piezoelectric actuator in such a way that the resulting stroke of the piezoelectric actuator actuates the control valve. An axial movement of a nozzle needle away from a valve seat is detected as a function of an increase in the voltage drop at the piezoelectric actuator. A termination of the movement of the nozzle needle is detected on the basis of an abrupt decrease in the voltage at the piezoelectric actuator.